fix(sessions): preserve original Event.id from raw_event in VertexAiSessionService (v1)

[chelsealong]

Ports #6531 to the v1 line, as requested by @ashubham there ("The fix works, can we please port to 1.x as well").

Problem

_from_api_event overwrote the ADK Event.id with the last segment of the Vertex event resource name, so the same logical event reported a different id depending on how it was observed — one value while streaming, another after a reload. Anything keyed on Event.id across a reload boundary saw them as two distinct events.

Fix

Stop clobbering id in the event_dict.update({...}) call and fall back to the resource name only when raw_event carries no id:

event_dict.setdefault('id', api_event_obj.name.split('/')[-1])

Older data without a raw_event id keeps the previous behaviour.

Evidence

Identical source change to #6531 (verified with a diff of both patches). Added the same regression test, test_get_session_preserves_original_event_id_from_raw_event, which puts a distinct id in raw_event and asserts get_session returns it rather than the resource name.

$ pytest tests/unittests/sessions/test_vertex_ai_session_service.py::test_get_session_preserves_original_event_id_from_raw_event
1 passed

# with the source change reverted (git stash), to show the test actually proves the fix:
1 failed

$ pytest tests/unittests/sessions/test_vertex_ai_session_service.py
33 passed

Branch is cut from current upstream/v1.

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Prepared with the help of Claude Code (Anthropic's AI coding assistant); I reviewed the diff and the test output before pushing.

[chelsealong]

@wuliang229 — the Unit Tests (Python 3.10) failure here doesn't come from this change.

The failing test is tests/unittests/telemetry/test_functional.py::test_metrics, and the same test fails on v1 itself — see 3d737f1, where it fails under Python 3.11. v1 was green at 99c1aa3 (22:12) and red at 3d737f1 (23:41), so it went red on its own branch within that window.

This PR only touches src/google/adk/sessions/vertex_ai_session_service.py and its test; nothing here reaches the telemetry path.

Happy to rebase once v1 is green again if that's easier for the import.

[chelsealong]

@wuliang229 — following up on your approval from 2026-08-07. The only thing that stalled this was the Unit Tests (Python 3.10) failure on the 08-07 run, which was tests/unittests/telemetry/test_functional.py::test_metrics and unrelated to this change (it failed identically on v1 at the time).

I've just updated the branch from current v1 (f29f66a356), so the old failure is gone; cla/google and header-check are green, and the CI run is waiting on maintainer approval to execute (action_required). If you can approve that run, this should be ready to merge as approved — no code has changed since your review.

I also closed #6996, which was an accidental duplicate of this same fix.